In a world laden with misery, it is the columnist's duty to find beams of joy that might leaven the gloom. So allow me to brighten your day with this fact: the cost of police pensions to the taxpayer has virtually doubled in real terms since Stephen Lawrence was killed in 1993.

A policeman's lot may not be a happy one (although for the top brass, at least, judging by Dame Elizabeth Filkin's evidence to Lord Justice Leveson on Monday, it had its moments). However, even the humblest ex-officer's lot is very heaven. They can retire at 50 and a half (and don't you love those extra six months for lending an air of stoic struggle?) on two thirds of their final salaries.

One to benefit from this quaintly old fashioned arrangement is John Davidson, star of yesterday's Independent report about corruption within the Met's investigation into Stephen Lawrence's death. Neil Putnam, a colleague of his who later turned supergrass, has sworn on oath that the then Detective Sergeant Davidson had a financial relationship with the drug-dealing father of one of the two men recently convicted of the murder. It further emerges that our flat-footed friend John Yates believed Mr Davidson to be sensationally bent before the Macpherson inquiry was conducted in 1999, but that the Met clumsily forgot to mention this to his lordship.

Yates has left the Yard over more recent examples of a laissez-faire approach than those involving Mr Davidson, and now supplements his pension by advising the police force of Bahrain – where else? – on human rights. Mr Davidson, meanwhile, departed for the sunshine back in 1998 (perhaps he hadn't turned 50 and a half, since he retired due to "ill-health"), and took the trail well trodden by Sarf London villains. He relocated to the Spanish island of Menorca, where he tops up his pension by running a bar he named Contrabandista – a cracking gag, possibly, given that his alleged speciality in his detective days was sharing their profits with the smugglers of cocaine.

Less ribcage-busting is the matter of how the Met decided that the way to treat an employee widely suspected of being a habitual criminal was not to make any attempt to bang him up for a long time; but to wave him off to sunny Spain with a pension luxuriously padded by you and me?

The likely answer is that – apart from fearing what else they might find if they lifted the lid on this one-man cesspool – they knew that prosecuting him would focus attention on the conduct of their Stephen Lawrence investigation. Presented with such evidence, Macpherson might have added a second finding to his one about racism. He may well have concluded that the Met was also institutionally corrupt... not because one dodgy detective was pally with a murder suspect's dodgy father (every barrel must have its rotten apples), but because it seems to have been common knowledge among his colleagues and superiors that Mr Davidson was a wrong 'un, and they turned a blind eye.

The full extent of Met corruption has become plainer since Mr Putnam's allegations surfaced in a BBC documentary in 2006, when the Met reacted with righteous indignation. "Angry Scotland Yard chiefs last night poured scorn on the BBC's film about the Lawrence murder investigation," one paper reported. "Top brass dismissed as 'a tissue of lies' allegations that former detective John Davidson was in the pocket of a suspect's criminal father. And they accused the BBC of 'utterly outrageous' film-making intended solely to cast slurs on the police."

The paper was The Sun, as you may have guessed, and the byline that of crime editor Mike Sullivan. Unlike Mr Davidson, Mr Sullivan has now been nicked, on suspicion of bribing public officials, and perhaps he received his briefing for that story while standing a senior copper one of those swanky meals to which Dame Elizabeth referred before Leveson. While compiling her report on the Met's relations with the media, she discovered that News International's indulgence of its hos at the Yard went beyond the tour through Michelinland. She also found that officers took many free tickets to prestigious sporting events from the Murdoch Mob.

There was "a very general view" among the lower ranks, she learned, that their guv'nors were "filling their boots" with NI nuggets, and of this they were resentful. No wonder, eking out the days until they turned 50 and a half on takeaway curries and Carlsberg while Assistant Commissioners were washing down slap-up meals with fine wines, if you'll pardon the lapse into Sunese, and sipping Pimms on Centre Court.It cannot have been easy for even the most honest among them not to become a little corrupted by, or at least inured to this twin-track culture.

Equine metaphor is mandatory this week, and if Mr Davidson was the bent cop version of the workaday hurdler trundling through the mud at Haydock Park, grabbing the odd £50 prize money from a dealer where he could, Commissioner Paul Stephenson was the cosseted Newmarket thoroughbred with his five-week Champney's freebie arranged by the old News of the World hand seconded to his press office. Sir Paul's "I was just a country boy" schtick at Leveson was hugely endearing. But, even to the naïve provincial who blunders into the country's top policing job like a smartly be-capped Chauncey Gardner, this much should have been evident: whether or not it involves brown envelopes or bags of white powder in Streatham car parks, corruption is corruption is corruption is corruption is corruption.

When we read about such practices in Italy or France, we nod sagely, and ask what else to expect from foreign police forces. Here, the public shrugs with indifference born of that lethally self-deceiving smugness about the imaginary cleanliness of British public life.

If there were any serious desire within the Met to reassure us that it has cleaned itself up, Mr Davidson would be on a plane to London in handcuffs by tonight. But the deeper issue is that of how a culture of secretive arrogance and complacent self-oversight enabled him to leave with his pension, and senior officers to sell their souls to a media group in plain view of their colleagues. If the Government fails to investigate, by setting a full judicial inquiry into institutional corruption within the Met, it will be a luminous national disgrace.